1. Field of the Invention
The invention herein relates to a kind of printer sharing device for computers which requires no external power supply and which is connected between computers and the printers via telephone cables the aforesaid printer sharing device can enable and disable itself automatically or be controlled by the software of the computer to switch between different printers: a single-chip microprocessor operates in series between the controller section of the computer and the controller section of the printers to handle data transfer and control code signal communications, and only one input/output (I/O) pin of the aforementioned microprocessor is required to enable the aforesaid communications; the controller section of the computer and the controller section of the printers are connected to the I/O pin of the microprocessor integrated circuit (IC) via wire conductor; when a printer is not in active use the conductance path office I/O pin is automatically transformed into a high resistance input state, wherein electrical conductivity is minimized; as a result, more than 25 computers can be connected to the invention herein and moreover, since the wire conductor in non-directional the printer sharing device can be connected to virtually any number of distribution points up to a distance of 1,200 feet to support a data flow rate of up to 80K bit per second.
2. Description of Related Art
With regard to conventional printer sharing devices for computers currently available one type is the switch box which has a mechanical switch; the switch box is connected to the 25-pin printer output port of a computer and the mechanical switch is operated manually to select a printer; another type of conventional printer sharing device has an electronic circuit that allows automatic switching and software can be used to select a printer. These two types of conventional printer sharing devices are basically centrally configured in that the printer sharing device is connected via the printer cable between the computer and the printer, so the length of the printer cable is limited, frequently to a maximum length of 25 meters and, furthermore, if there is an excessive number of computers and printers, then the installation of the required printer cables is inconvenient; therefore, the aforemention two types of printer sharers are frequently limited to manufacture in a four-to-two, two-to-one and other printer-to-computer distribution ratios to enable practical usage.
At present, there is a relatively improved method of printer sharing that is indicated in FIG. 1, wherein an automatically switching printer sharing device supports linked distribution; the aforesaid type of automatic printer sharing device utilizes a hardware-based circuit to output printer control code and data signals which are transferred in series through a four-conductor telephone cable to a receiver adapter at the printer, the aforesaid receiver adapter then transfers the inputted control code and data signals into the printer; when the aforementioned cable is already in the process of transferring signals, then the aforementioned circuit automatically disables computer output to the printer. The advantage of this type of printer sharing device is that a relatively high number of computers can be connected and that the installing of the requisite connector cables is convenient.
However, the capability of the conventional linked-distribution automatically switching printer sharing device is still subject to certain limitations because the printer relies on handshaking to coordinate data transfers flowing between the computer and the printer; as a result, a minimum of three conductors must be present in the interfacing cable (transmit line, receive line and ground line), and only one printer can be connected with four-conductor cable; some types of linked-distribution automatically switching printer sharing devices now available on the market can be connected to many printers and with regard to the printer settings, the software of the computer selects the printer; however, only one printer can be utilized at a time because when the connecting cable is conducting communications signals, the other printers are disabled. In addition, when the high-speed data transmission distance is long, if the impedance is not matched and there is no sheilding against spurious electromagnetic emissions interference (EMI), it is extremely easy for data transmission errors to occur; ordinary telephone cable has no effective shielding against spurious EMI and, furthermore, since the number of computers connected may vary, it is difficult to determine the proper matching impedance; therefore, the transmission distance of the conventional printer sharers of this type now in current use is very short and if the transmission is lengthened, it is necessary to add a repeater circuit, which has a low transistor-to-transistor logic (TTL) signal transmission rate of 50K bit per second; the aforesaid transmission rate is too slow for high-speed printers or buffer-equipped printers.
In addition, all of the aforementioned conventional printer sharing devices accomplish automatic functions through the utilization of a high number of digital ICs, IC chip sets and other electronic components to increase capability; some printer sharers, especially units with long-distance transmission capability or that can be connected to a relatively large number of computers, even require an external power supply. Moreover, since the cable from the printer sharer has a fixed transmission direction the printer must be connected at the opposite terminal point, making it impossible to connect according to preference and also limiting the connection capacity to only 20 computers.